An Early Jurassic oligohaline ostracod assemblage within the marine carbonate platform sequence of the Venetian Prealps, NE Italy

Micropalaeontological investigation of one of a number of black shale horiz ons within the carbonate platform sediments of the Calcari Grigi Formation (Trento Platform, NE Italy) has yielded the first recorded oligohaline, ear ly Jurassic, ostracod assemblage. The shale is dated as late Sinemurian on the basis of large benthic foraminiferal biostratigraphy in the sediments a bove and below. The shale is devoid of foraminifera and ammonites, which su pports a non-marine context. Three previously unknown ostracod species are identified, however, the assemblage is dominated (>95%) by a single taxon w hich is erected as a new genus and species (Phraterfabanella tridentinensis Whatley and Boomer gen. et sp. nov.) which further supports the interpreta tion of a "stressed" environment. The two remaining ostracod taxa are assig ned to Klieana and Limnocythere both considered to represent non-marine or very low salinity conditions. This new genus includes probable members from the Rhaetian of Hungary and France as well as from the Liassic of France. The new genus is shown to be one of the earliest representatives of the Cyt herideidae, a family which survives to the present and includes the modern pandemic, euryhaline species Cyprideis torosa. The discovery of such an ass emblage within an otherwise marine sequence indicates temporary, physical i solation from marine influence. Sedimentological, faunal and geochemical ev idence suggests that the organic rich shale may indicate high organic carbo n supply via continental runoff in this tropical palaeogeographical setting . Resulting dysaerobia within the sediment would have created a particularl y stressed environment. (C) 2001 Elsevier Science B.V. All rights reserved.